Steel pipes laid underwater, particularly in the sea, are currently used for carrying oil obtained from marine oil wells or for carrying other liquids and gases. These pipes, which typically have outside diameters of from 12 inches (30.5 cm) to 48 inches (121.9 cm), are manufactured in sections, generally 40 foot (12.2 m) lengths, and are generally provided with a reinforced concrete coating. The function of the pre-formed concrete coating on the steel pipes is not so much to protect them from corrosion (since the steel piping is provided with its own corrosion protection) as to weigh the pipes down on the sea-bed and to protect the pipes against impact damage, such as might be caused by a colliding trawler board. It has been estimated that a pipe joint may receive, on average, five such impacts during the expected lifetime of the pipe.
The sections of pipe are welded together, to form a pipeline, on a so-called "lay barge" and the uncoated area is protected by the application of either a marine mastic asphalt or a resin-based composition. In general, the procedure on the lay barge comprises the steps of welding together two lengths of pipe; conducting an X-ray inspection of the weld; placing a mould around the welded joint; filling the mould with the protective material; if appropriate, stripping off the mould; and passing the welded pipe over the "stinger" (a device comprising rollers situated at the stern of the barge) and into the sea.
The materials used hitherto for protecting the weld areas have certain disadvantages. Thus, marine mastic asphalt must be heated, which not only increases the cost of the operation but also generally requires the mould to go into the sea in place because the mastic does not cool down sufficiently in time to allow demoulding. The moulds can be hazardous on the sea-bed, since they may snag fishing lines or the like. The resin-based systems can be placed in reusable moulds that can be removed about 10 minutes after filling but give rise to environmental disadvantages: in particular, one of the starting materials contains isocyanate and must not be allowed to go overboard. Furthermore, the mixing and placing equipment used with the resin-based systems has to be flushed with solvents after each moulding operation, thereby giving rise to additional environmental and explosion risks as well as inconvenience during operations.
In order to improve operational efficiency, and to reduce the hazards associated with the prior art, it would be desirable to employ a protective material that fulfils the following requirements, namely a material that does not require heating to elevated temperatures; that does not entail the use of large quantities of toxic resins or flammable solvents; that can be placed readily in a mould and that allows the mould to be stripped off within a period of about 6 minutes; that attains sufficient flexural and compressive strength within about 10 minutes after the filling of the mould to be passed over the stinger; and that attains a sufficient strength to withstand an impact test as used by the Corrosion and Protection Centre Industrial Service (CAPCIS) at the University of Manchester Institute of Science and Technology (UMIST), which test involves an impact with a swung 2.68 tonne chisel-ended weight at 7 knots (13 km/h). 2.68 tonnes is the current maximum weight of a trawler gate and 7 knots is the maximum speed at which it is currently permitted to travel.
It is believed that no previously known hydraulic cement-based compositions would have the required rapid setting and strength development, the required impact resistance, and the ability to meet other demands of the above-described use. Thus, the concrete mix would need initially to be sufficiently flowable to permit satisfactory filling of the mould placed around the pipe joint. The concrete would also need to withstand the severe thermal stresses due to emplacement around a pipe joint still hot (typically 110.degree.-120.degree. C.) from the welding operation followed by immersion shortly afterwards in cold seawater (typically 5.degree. C.): conventional concrete subjected to such a thermal shock would be expected to undergo multiple cracking and to lose quickly its integrity.
The concrete should exhibit shrinkage properties compatible with those of the freshly welded steel pipe, which will cool quickly upon entry into the sea; furthermore the concrete should be dimensionally stable despite immersion in seawater.
U.S. Pat. No. 4,377,977 (Wurster) discloses a moldable, uniform mixture comprising 120-190 parts by weight of silica sand aggregate, 40-65 parts of metal fibre, in particular steel fibre, 80-175 parts of an expansive cement containing a Portland Cement and an expansive component, and 30-75 parts of water, said mixture having a slump of about 5.5 to about 6.5 inches (140-165 mm). The concrete obtained upon curing the said mixture can withstand an attack with an acetylene torch without flaking, spalling or exploding and is also resistant to attack with a hammer, chisel, drill or cutting implement; the said concrete is therefore suitable for incorporation in a safe or vault structure. Other materials, such as granite chips or a combination of latex binder and glass fibre may also be included in order to improve impact resistance. The said expansive component must be present in an amount that is at least sufficient to compensate for the shrinkage of the said Portland cement and to impart expansive and self-stressing properties to the Portland cement when the said component is hydrated upon curing: preferably, the expansive cement contains 90-70% of Portland cement and 10-30% of an expansive component consisting for the most part of calcium sulfoaluminate (C.sub.4 A.sub.3 s) in the form of a ternary system with extractable associated lime (CaO) and extractable associated anhydrous calcium sulfate (CaSO.sub.4). This U.S. patent indicates that a preferred expansive, shrinkage-compensating cement is that disclosed in U.S. Pat. Nos. 3,155,526 and 3,251,701.
The characteristics exhibited by the concrete mixes disclosed by Wurster render them unsuitable for the preferred use of the compositions of the present invention. Thus, Wurster's compositions are required to have the stated slump values in order to ensure that the mouldable mixture can be vibrated into various cavities and interstices of the safe cavity (U.S. Pat. No. 4,377,977, column 7, lines 7-10 and 18-33); such characteristics would be unsuitable for coating the weld areas of steel piping to be laid in the sea, where demoulding has to be effected within a very short span of time. Furthermore, Wurster's compositions undergo an expansive reaction in the first few days of concrete curing (U.S. Pat. No. 4,377,977, column 6, lines 47-48); however, such expansion in a pipe weld coating could cause separation of that coating from the steel piping (which would have contracted upon cooling) thereby considerably weakening the whole structure and reducing the corrosion resistance.